1) Field of the Invention
The present invention relates to a method for drying resin-used electronic parts, and specifically, to a method for drying resin-used electronic parts wherein no quality deterioration due to oxidation and degradation of a metal surface in a drying step of resin-used electronic parts occurs.
Examples of the resin-used electronic parts of the present invention include printed wiring boards such as one side printed wiring boards, both sides printed wiring boards, multilayered printed wiring boards, flexible printed wiring boards, metal core printed wiring boards, discrete wire wiring boards, electrically conductive paint printed wiring boards, ceramic base printed wiring boards, electric resistor printed wiring boards, etc., electric semiconductors, typically including IC and LSI, electric resistors, electric condensers, etc. Further, the method of the present invention is applicable to not only finished products composed of the resin-used electronic parts, but also intermediate products and raw materials thereof.
2) Prior Art
Printed wiring boards perform an important role in mounting since electronic parts are loaded and electrodes are connected. With recent tendency of change to a smaller type of electronic device, its high performance, its high reliability and its low cost, high density and high integration of wiring in electronic devices have been developed. Thus, dimension stability of printed wiring board, cleanliness of metal electric circuit surface, high reliability of printed wiring board during mounting of electronic parts has been required. Therefore, when electronic parts are mounted, moisture and low boiling point organic substances contained in a printed wiring board and remained liquid in washing after coating and etching are usually removed and the printed wiring board is dried in advance to prevent swelling and curvature. For example, a printed wiring board is dried in advance at a high temperature of 100.degree. C. or above for several hours. However, in the drying step, there sometimes occurred such troubles that a surface of metal electric circuit such as copper, aluminum on a printed wiring board was oxidized to cause discoloration of metal portion of the electric circuit; adhesiveness in soldering or wire bonding of electronic parts was lowered and curvature of a printed wiring board due the difference of thermal expansion coefficient between metal and resin was generated.
Also in electronic parts including electric semiconductors, electric resistors, electric condensers, etc., epoxy resin or polyimide resin has been used in package thereof. Thus, in order to remove moisture and low boiling point organic substances contained in the resin portion, electronic parts are dried in advance at a high temperature of 100.degree. C. or above for several hours. Further, recently, even when a polyimide film, as in LOC type electric semiconductor (Lead on Chip), is used as insulating material, drying treatment is usually conducted since when polyimide film with high hygroscopicity absorbs moisture, its dimension is varied; degradation due to corrosion for a metal in a lead part occurs and thus mounting is impeded. However, in such drying treatment method, a metal surface in a lead part has been sometimes oxidized to cause discoloration and adhesiveness was sometimes lowered in the step for soldering or wire bonding of electronic parts.
As methods for preventing oxidation and degradation of metal surface in such drying step of electronic parts, it was attempted to lower the treating temperature or to shorten the drying time. However, although it would be expected to some extent to prevent oxidation of metal surface, there often occurred such problems that it was not perfect to prevent oxidation or drying and removal of evaporating components were insufficient, resulting in swell or crack in soldering during mounting of electronic parts. In drying treatment used a vacuum dryer, although it could be expected to prevent oxidation of a metal surface and to shorten the treating time, there was a problem in cost since expensive exclusive apparatus was necessary. Further although there was also a method for drying electronic parts comprises immersing resin-used electronic parts in advance in a readily vaporizable organic solvent such as flon (CFCs) to replace moisture with the organic solvent and then drying, there occurred environmental problems and a problem in danger of ignition and furthermore a problem that moisture in the atmosphere was re-absorbed during cooling after drying. Thus, under the present situation, no simple and easy method for drying resin-used electronic in which quality deterioration due to oxidation and degradation of a metal surface can be sufficiently prevented, has yet been found.